Big Idea:
Problem solving requires students to identify the task at hand and conditions for a suitable solution before engaging in solving the problem.

This will be the first guided inquiry laboratory experience my students have received this year. In the guided inquiry setting, students are given a desired outcome, and significant scaffolding to help them design an experiment or process to reach that outcome. We have used guided inquiry in computer simulations and via the POGIL activities, but not in the lab. In our remaining curriculum, students will be expected to perform at least three additional inquiry labs.

This lesson is the first of three, today the students will be performing the background calculations prior to planning their investigations. This occurs after students have had practice with mass to mass calculations in a direct instruction setting. However, this will test student ability to transfer their understanding to a practical, hands on scenario. I have been telling students as we have been learning about the mole and stoichiometry that they will have to prove their understanding in the lab. The prior knowledge expected from the previous lessons is the ability to balance chemical equations, calculate molar mass, and perform multi-step stoichiometry calculations.

This is the first of two stoichiometry labs the students will do. This follows the AP guided inquiry model of a structured activity which is followed by a less structured scenario where the students will need to design their procedure and data collection. This experiment is strongly guided, with all students using the same reaction, and being able to collaborate and check ideas. For this activity, I am having the students work with their table mates to ease the process and keep them with people they are familiar with.

I am using this context of cloud seeding because the reaction between lead (II) nitrate and potassium iodide is one they have seen and performed. When I first did the lab in 2012, it was very relevant as it was directed towards the 2008 Beijing Summer games, and with the 2012 London Olympics coming up, students were interested in how it could be used. Although the California drought doesn't affect my students directly, it provided a real world reason to do chemistry, which is something that our local curriculum overall is missing.

I include the video to help students wrap their heads around what cloud seeding looks like, and how it was going to be used to control the rain in London in 2012. I found the year I began including the video, the number of questions about what the process of cloud seeding entails decreased significantly, allowing us to focus more on the embedded stoichiometry.

This lesson addresses a plethora of standards:

HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

When students enter the class, I explain a slight change in plans. I had told students the day before that I would be assigning lab partners for the first time this year today. However, I had a change of mind and decided to let them work with their normal tables. To understand why I made that change, check out the attached reflection.

I pass out the paper and ask students to read the introduction paragraphs on the Save the Day Lab sheet. While students are reading about how lead (II) iodide is used for cloud seeding, I take attendance and begin to circulate the room.

When students have finished reading, I ask if they have heard of cloud seeding before, and where they might have heard it. Since students are likely to have not heard of it before, I explain that it was used to clear air pollution before the Beijing Olympics in 2008, and to cause rain to fall away from London for the 2012 Olympics. I then show this YouTube clip to give them a little background.

When students have finished viewing the video, I ask them to read back over the first paragraph and identify which chemical we need to create for cloud seeding. Since we did not teach chemical nomenclature this year, I am fine with responses of either PbI2 or Lead II Iodide, depending on how students interpret the paragraph.

I then direct them to the chemical reaction, and ask them to break it down and fill in the four blanks beneath the equation. Students correctly identified the reactants and products, but struggled remembering that the precipitate is the solid product.

Resources

Now that students have identified the reactants (lead II nitrate and potassium iodide) and their products (lead II iodide precipitate and potassium nitrate) we move onto determining their objective.

I ask students to review the 2nd paragraph of the introduction, and to write their goal in the box provided. While students are writing, I circulate the room to see what they are discussing and writing. When most groups are finished, I ask if anyone would be willing to share their objective. Ideally, student responses will be "to determine the mass of each reactant needed to produce 1 kg of PbI2"

Student responses may vary, and might specifically identify each reactant. They also may not include the 1 kg measurement in their response. This is a very important piece. I have been training the students to look for three things together in stoichiometry problems to know where to start:

A number

A measurement unit

A chemical formula

Here, representing 1 kg of PbI2 is the culmination of applying simple exercises to a larger, contextual problem.

Next, I point out the six jumbled steps to reaching their objective.

Find moles of reactant

Balance chemical equation

Find mass of product

Find mass of reactant: KI

Find moles of product

Find mass of reactant: Pb(NO3)2

I direct each group to put the steps into the proper order, thinking about what they have learned so far about performing stoich problems. When they are deciding the order, they need to justify why the step is in that place in the space to the right of the step.

The correct order is:

Balance chemical equation

Find mass of product

Find moles of product

Find moles of reactant

Find mass of reactant

The masses of the reactants can be done in either order, so I did not differentiate above. Steps 2-5 should look familiar to students who have been using the factor label method to solve mass to mas conversion throughout the week. While students are working, I am circulating the tables and checking into their conversations and answering questions. If students are struggling, I direct them to their practice worksheets from earlier in the week to determine how we perform this.

This is a student sample including both their objective and sorted calculation steps.

When a group is set, I check their order and tell them to go ahead and follow their steps. Students will need to balance the equation, and then set up the dimensional analysis to find the mass of each reactant. To save time, at this point I will stop and write on the board that 1 kg = 1000 g, so they should begin with 1000 g. I will not identify the chemical for them leaving that to their partnerships.

Depending on the group dynamics, they may split up their tasks, or they may choose to all work through them together.

This student did the balancing of the equation for his group:

This student did the stoich calculations with help from her group. The group did the molar masses and balanced the equation. You can see they caught her error in the second calculation that the mole ratio was initially incorrect.

When students are finished calculating, I ask them to staple the papers of the group together and turn them in. This step took much longer than I expected of students, so we will scale the amounts down and design the procedure tomorrow.

Similar Lessons

Big Idea:
Atoms are not destroyed or created in chemical reactions; they are rearranged. This means that a reaction must have the same amount of atoms of each element on both sides of a chemical equation.